The invention relates to a method of coating a luminescent material with a layer of a metal oxide M2O3 in which a metal M is chosen from the group formed by Y, Al and La, in which a compound of M is deposited on the luminescent material by means of homogeneous precipitation from a watery solution.
The invention also relates to a luminescent material obtainable by such a method and to a discharge lamp equipped with a luminescent screen comprising such a luminescent material.
A method as mentioned in the opening paragraph is known from WO 96/05265. Coated luminescent materials show a relatively high stability in the watery slurry that is used in the lamp manufacturing process and an improved maintenance of the light output. A drawback of the known process is that many luminescent materials (for instance materials having a silicate host lattice) are sensitive towards hydrolysis at the relatively low pH values at which the homogeneous precipitation is taking place. Additionally it has been found that the optical properties such as reflectivity and quantum efficiency of other luminescent materials that do not hydrolize in water are adversely effected by a low pH value.
The invention has for its object to provide a method of coating a luminescent material with a metal oxide by means of homogeneous precipitation from a watery solution in which hydrolysis of the luminescent material and degradation of its optical properties are to a large extent prevented.
According to the invention, a method as described in the opening paragraph is for this purpose characterized in that, in that order, a watery suspension of the luminescent material is prepared, a watery solution containing a complex of M and an organic chelating agent is added to the watery suspension, the luminescent material is separated from the watery suspension and is dried and heated.
It has been found that the organic chelating agent prevents the precipitation of M(OH)3. As a result the homogeneous precipitation can take place at a relatively high value of the pH. Accordingly hydrolysis and degradation of the luminescent material are strongly suppressed.
Good results have been obtained with ethylenediamine, trispyrazolylborate, diglyme, benzoic acid, crown ethers, polyphosphates and triazacyclononane as the organic chelating agent. More in particular good results have been obtained with ethylenediamine tetraacetate.
Good results have also been obtained in case the pH of the watery suspension is in the range 8-10 and the pH of the watery solution is in the range 6.5-7.5.
Luminescent materials obtained by means of a method according to the invention typically have a fine grainy coating consisting of a thin layer of spherical M2O3 nanoparticles with a diameter smaller than 30 nm. The coating can be distinguished from coatings that were applied by means of chemical vapor deposition (CVD) by means of scanning electron microscope (SEM) or transmission electron microscope (TEM) analysis. The coating properties can also be reflected in macroscopic phosphor features such as slightly enhanced light output due to cleaner surface with a higher scattering compared to coatings applied by means of CVD.
A method according to the invention is particularly suitable for coating luminescent materials having a silicate host lattice, since these luminescent materials hydrolize very easily in a watery solution having a relatively low pH. More in particular the method has proven to be very suitable for the coating with La2O3 of BaSi2O5 activated with lead, (Ba, Sr)2MgSi2O7 activated with lead and Zn2SiO4 activated with manganese.
Luminescent materials coated by means of a method according to the invention were found to be very suitable for use in the luminescent screen of a discharge lamp, more in particular a low pressure mercury discharge lamp.